Microprocessor designs use many different types of logic circuits which provide a desired logic level output signal in response to one or more logic level input signals. Each logic circuit input signal comprises a signal at one of two alternate voltage levels, each alternate level corresponding to a logic state. A "low" level voltage signal represents one logic state while a "high" level voltage signal represents the alternate logic state. The voltage range for a valid "low" signal as well as the range for a valid "high" signal are determined by the types of semiconductor devices used to build the particular logic circuit.
An exclusive OR circuit (commonly referred to as an "XOR" circuit) is a common logic building block used repeatedly in microprocessors. The output of an exclusive OR circuit comprises a logical "high" output only when the logic state of a first input signal is unequal to the logic state of a second input signal. Thus, an exclusive OR circuit produces a "high" output only when the first input to the circuit is at a logic state which is opposite to the logic state of the second input. When the first input signal and the second input signal to the exclusive OR circuit are equal, either both "low" or both "high", the circuit produces a "low" level output signal.
As with all logic circuits used in a microprocessor, the performance of the exclusive OR circuit is critical to the cycle time of the microprocessor. The physical size of the exclusive OR circuit is also important because it is used so frequently in microprocessors. Also it is important that the exclusive OR circuit dissipate a minimum amount of power in providing the desired logic function.
An exclusive OR circuit and other logic circuits may be connected together in a cascade arrangement such that the output of one circuit provides the input to another circuit. It is important in this cascade circuit arrangement that the logic circuits be synchronized so that each circuit provides the desired logic output in a timely fashion for use by the next logic circuit in the series. Each circuit must produce and maintain the desired logic level output so that following logic circuits may perform the intended logic function.